John Felde | University of Maryland | USA

Excitement from IceCube

After a rather long hiatus (I was writing my PhD dissertation), I am getting back into the habit of posting about interesting things happening in particle physics. Since finishing my degree at UC Davis, I made an arduous cross country drive to start a new adventure as a postdoc at the University of Maryland working on the IceCube neutrino experiment at the South Pole. I have joined this collaboration at a particularly exciting time since the full detector was completed in May of 2011.

COVER Hit distribution (red, early; green, late) of a neutrino interaction with the Antarctic IceCube neutrino detector on 14 July 2011. Light from this transfer of 250 teraelectron volts of energy fills a sphere 600 meters across. This event, among the highest-energy neutrino interactions ever observed, forms part of the first evidence for a high-energy neutrino flux of astrophysical origin.

Back in June of this year, two neutrino events were reported with energies slightly above 1 PeV (peta-electronvolt). To put this number in context, the protons circulating in the Large Hadron Collider (LHC) at CERN have energies of about 4 TeV (tera-electronvolt) each. A PeV is 1,000 times greater than a TeV. Although we would love to be able to produce these higher energies at colliders like the LHC, it simply isn’t feasible at this time. As a result, we must rely on nature to produce these high energy particles for us, and hope that she flings a few our way so we can detect them. This is the job of the IceCube detector, a huge, 1 cubic kilometer, neutrino detector instrumented deep within the Antarctic ice. The enormous size is necessary since few of these particles are produced at such high energies, and even then the neutrino interaction probability is miniscule. Unfortunately, the physicist has no control over nature, nor physics, and so our only recourse is to build big! For those interested in more details about the detector, see the website at the University of Wisconsin – Madison here.

Today the collaboration reports findings from a new neutrino search published in Science. The new search includes neutrino events at lower energies as well, down to about 30 TeV. The results of this search indicate that it is highly unlikely that these neutrinos were produced by any mechanism at Earth. Many high energy neutrinos are produced in Earth’s atmosphere, but not this many, and not at these energies.

Of particular interest to the community is a very fundamental question: “Where do these particles come from anyway?” Since the neutrino interactions preserve some information about the neutrino’s direction, the hope is that these neutrino events will all be coming from a particular place in the universe. Looking for this, the results are tantalizing. Since not all of the events provide exact position information, our best guess of the particle’s direction can be a little fuzzy. So far, however, the most significant clustering of events can be seen below in the full skymap (bottom left side). This location roughly corresponds to the center of our galaxy, but the fuzziness of the event locations does not permit us to say where exactly these neutrinos are coming from.

Luckily, the detector continues to collect additional neutrino events, even possibly as you read this. Our fingers are crossed that more events will be detected in this regime, filling in our understanding of extraterrestrial neutrinos and the cosmos in general.

Skymap of the IceCube neutrino events. The purple regions indicate more likely locations of neutrino sources (darker is more likely). The plane of the galaxy is shown as a grey line, and the center of the galaxy is denoted by a filled grey square (near the event marked #14). The best guess for each event’s location are indicated with either a + (shower like events) or a X (muon tracks).